It is of great importance in many fields of technology to be capable of constructing a three-dimensional illustrative representation from a series of linear data resulting from various projections (i.e. line-of-sight measurements) taken of the matter one desires to reconstruct. For instance, by employing X-rays to provide a three-dimensional image of a human body, or part thereof, it is commonly known to pass X-rays through the body in a number of different directions and to measure the absorption of the X-rays.
Passing a planar beam of radiation through an object and detecting the amount of absorption within a cut of the object results in an essentially two-dimensional object being projected onto a one-dimensional image. Similarly, passing an unfocused beam of radiation through a three-dimensional body and detecting the amount of absorption within the body results in a three-dimensional body being projected onto a two-dimensional image. This results inevitably in superimposition of information and resulting loss of the information. Complex techniques have to be employed if one wishes to perform an examination with greater sensitivity to spatial variations in radiation absorption and with less severe superimposition effects.
In a computerized tomography (CT) examination method known as helical scanning a source of a radiation beam and a detector (photographic film or digital detector) are arranged for irradiating the object to be examined by the radiation beam, and for detecting the amount of radiation passed trough (i.e. not absorbed or scattered off) the object. The radiation source and the detector are revolved along a circular or other path around the body, while the object may be moved linearly in a direction orthogonal to the plane of the revolution, and readouts of the detector are performed at several positions of the revolution of the radiation source and the detector, and optionally at several positions of the linear movement of the object. Alternatively, the radiation source and the detector are revolved in a helical fashion, while the body is kept still. A three-dimensional reconstruction process of the body is then performed, wherein different structures of the body, e.g. soft tissue, bone, liquid-filled cavities, etc., become distinguishable as these structures show different absorption.
Detection devices for detecting the radiation in tomographic apparatuses of the kind depicted above include various kinds of scintillator-based detectors, gaseous ionization detectors, and solid-state detectors.